/Engine/Source/Runtime/Engine/Classes/Components/DecalComponent.h
Sets the decal's fade start time, duration and if the owning actor should be destroyed after the decal is fully faded out. The default value of 0 for FadeStartDelay and FadeDuration makes the decal persistent. See DecalLifetimeOpacity material node to control the look of "fading out."
void SetFadeOut ( float StartDelay, float Duration, bool DestroyOwnerAfterFade )